Often in communications, it is desirable to know the relative position of a radio with respect to its communication base (also referred to herein as a “base station” or simply “base”). In one example application, this information is useful in systems related to “presence”. The term presence generally refers to information about a user's ability or willingness to communicate. In the prior art, the concept of using presence in communication systems is often applied in instant messaging systems. Presence is also used in other network communication systems, such as the Microsoft Unified Communication Service. As applied to the field of headsets, typical presence information may include, for example, whether the headset is being worn by the user, the proximity of the user to the base station, other usage information related to the headset, and whether the user desires to be called.
In some applications, the position information required may be a NEAR/FAR binary state, namely either a closer proximity (referred to herein as a “near” or “NEAR” state) or relatively farther proximity (referred to herein as a “far” or “FAR” state), with the threshold between the two states set by the application. One indicator of relative position is received radio signal strength. Often a number is assigned to this strength and is referred to as the received signal strength indication (RSSI). Most manufacturers who report RSSI generally estimate the received signal power at the antenna either by direct measurement, or digital signal processing, and report a monotonically increasing number with respect to this power. The number is often calibrated to track power linearly and report the value in dBm.
Received signal strength depends on transmit power level, the direct line of sight distance between transmitter and receiver and any reflected radio waves received (multi-path). As the direct line of sight distance increases (for fixed transmit power), the received amplitude decreases (square law for free-space, no reflections). In some situations, the direct path is blocked and only reflections are received.
When the direct path is not blocked, deep fades can be caused by reflectors at or beyond the first Fresnel zone. Fresnel zones are ellipsoids with transmitter and receiver at the foci and the surface defined by all paths that are an odd-multiple of a half-wavelength farther than the direct path between transmitter and receiver, causing cancellation (assuming no phase shift at the reflector).
If the reflector is at an even multiple of a half-wavelength, the direct and reflected waves can constructively interfere (again assuming no phase shift at the reflector) and the received amplitude is twice as large (6 dB) as the direct path alone. In general, accurate predictions of real situations are difficult, but one can state in general that the actual Received Signal Strength Indication (RSSI) can vary by +6 dB to −infinity depending on the reflector configuration.
When the direct path is blocked, fades can also occur. Simple analysis is more difficult, but statistical models have been made, and again generally RSSI will decrease with distance. Often one reflector dominates, and the simple analysis above for direct path fading can be used.
If RSSI is measured at the fade frequency, the estimated position based on RSSI can be very inaccurate. If a threshold is used to determine NEAR/FAR, it can be triggered at a close range. FIG. 6 is a graph illustrating a simplified RSSI profile 602, an average RSSI 604, and peak RSSI 606 as a function of frequency at a near range of approximately 17 inches to the base station (the 7th Fresnel zone for 2.45 GHz ISM band.)
FIG. 7 is a graph illustrating an RSSI profile 702, average RSSI 704, and peak RSSI 706 as a function of frequency at a FAR range of approximately fourteen feet to the base station (the 71st Fresnel zone for 2.45 GHz ISM band.). As shown in FIG. 7, the farther away the headset from the base station, the more frequency sensitive the fades become, but the less bothersome if averaging is used. FIGS. 6 and 7 are simplified RSSI models involving a direct signal and a single bounce signal, totally reflected. As shown in FIG. 6, a peak RSSI signal value 606 would be within 6 dB of the line of sight value alone (0 dB), whereas an average RSSI value 604 could be more than 15 dB or more below the line of sight value, even at close range, causing a false FAR report.
If the headset is sufficiently FAR away, the occasional places where constructive interference occurs between line of sight and a strong reflection will yield approximately 6 dB above a square law estimate. There are also changes due to polarization of antennas and reflection effects. Again, statistically, one would expect to see times when polarization line up and others where there is fading due to cross-cancellation. In headset applications, generally polarization cannot be guaranteed due to how the user wears the headset, and a varying reflective environment.
In the prior art, one way to improve RSSI measurements is to measure it across several frequencies. This is done intrinsically by frequency-hopping systems such as Bluetooth. In the prior art, the RSSI is often reported as the average RSSI value over packets sent at each of the hopping frequencies. However, using the average RSSI value when there is multi-path is problematic, as the fade can drag down the average RSSI value significantly. Close-in, the majority of the band can be in fade.
OFDM also characterizes RSSI over the band. By using pilots at different frequencies, it senses the fades and can provide equalization to transmitted data. But this information has not typically been used for ranging.
As a result, there is a need for improved methods and apparatuses for headset ranging relative to its radio base.